Compact balanced bandpass filter using interdigital line resonator with high common‐mode noise suppression

A novel balanced microstrip bandpass filter (BPF) using compact interdigital line resonator (ILR) is proposed in this article. The presented balanced BPF has shown good common‐mode (CM) noise suppression, while the size is reduced. For design purposes, the differential‐mode (DM) and CM equivalent circuits of the ILR are given. The DM resonator has a smaller size compared to the conventional quarter‐ and half‐wave resonators. In addition, the CM resonant frequency is much higher than the DM resonator frequency in the ILR. Based on this property, the CM response can be strong suppressed over the DM operating frequency range. A fourth‐order balanced BPF with a size of 23 × 22.5 mm2 and 50 dB CM noise rejection within the passband is demonstrated and measured.